Solid oxide fuel cell and separator

ABSTRACT

A solid oxide fuel cell is formed by arranging a fuel electrode layer and an air electrode layer on both surfaces of a solid electrolyte, respectively, a fuel electrode current collector and an air electrode current collector outside the fuel electrode layer and the air electrode layer, respectively, and separators outside the fuel electrode current collector and the air electrode current collector. In a first embodiment, a fuel gas and an oxidant gas are supplied from the separators to the fuel electrode layer and the oxidant electrode layer, respectively, through the fuel electrode current collector and the air electrode current collector, respectively. Each separator is formed by laminating a plurality of thin metal plates at least including a thin metal plate in which a first gas discharge opening is arranged in a central part and second gas discharge openings are circularly arranged in a peripheral part, and a thin metal plate with an indented surface. Gases discharged from the separators can be supplied to entire areas of the electrode layers through the current collectors, so that electric power generation can be performed.

This application is a divisional of U.S. application Ser. No.10/506,526, filed Sep. 3, 2004 now U.S. Pat. No. 7,201,991.

TECHNICAL FIELD

The present invention relates to a solid oxide fuel cell, morespecifically to a separator in a planar solid oxide fuel cell in whichintroduced gas is supplied to an entire area of a current collector tothereby equalize an imbalance in an electrode reaction, and animprovement of electric power generation efficiency is achieved.

BACKGROUND ART

Development of a solid oxide fuel cell, having a laminate structure inwhich a solid electrolyte layer made of an oxide ion conductor issandwiched between an air electrode layer (oxidant electrode layer) anda fuel electrode layer, is progressing as third-generation fuel cell foruse in electric power generation. In a solid oxide fuel cell, oxygen(air) is supplied to an air electrode section and a fuel gas (H₂, CO andthe like) is supplied to a fuel electrode section. An air electrode anda fuel electrode are both made to be porous so that gases can reachinterfaces in contact with the solid electrolyte layer.

Oxygen supplied to an air electrode section passes through pores in theair electrode layer and reaches a neighborhood of the interface incontact with the solid electrolyte layer, and in that portion, theoxygen receives electrons from the air electrode to be ionized intooxide ions (O²⁻) . These generated oxide ions move in the solidelectrolyte layer by diffusion toward the fuel electrode. The oxide ionshaving reached the neighborhood of the interface in contact with thefuel electrode react with fuel gas in that portion to produce reactionproducts (H₂O, CO₂ and the like), and release electrons to the fuelelectrode.

The electrode reaction when hydrogen is used as fuel is as follows:

Air electrode: ½ O₂+2e⁻→O²⁻

Fuel electrode: H₂+O²⁻→H₂O+2e⁻

Overall: H₂+½ O₂→H₂O

Because the solid electrolyte layer is a medium for migration of theoxide ions and also functions as a partition wall for preventing directcontact of the fuel gas with air, the solid electrolyte layer has adense structure capable of blocking gas permeation. It is required thatthe solid electrolyte layer has high oxide ion conductivity, and ischemically stable and strong against thermal shock under conditionsinvolving an oxidative atmosphere in the air electrode section and areductive atmosphere in a fuel electrode section. As a material whichcan meet such requirements, generally a stabilized zirconia (YSZ) thatis added with yttria is used.

On the other hand, the air electrode (cathode) layer and fuel electrode(anode) layer need to be formed of materials having high electronicconductivity. Because an air electrode material is required to bechemically stable in an oxidative atmosphere of high temperatures around700° C., metals are unsuitable for the air electrode, and generally usedare perovskite type oxide materials having electronic conductivity,specifically LaMnO₃ or LaCoO₃, or solid solutions in which part of an Lacomponent in these materials is replaced with Sr, Ca and the like.Moreover, the fuel electrode material is generally a metal such as Ni orCo, or a cermet such as Ni—YSZ or Co—YSZ.

A solid oxide fuel cell is classified into a high temperature operationtype operated at high temperatures around 1000° C., and a lowtemperature operation type operated at low temperatures around 700° C. Asolid oxide fuel cell of the low temperature operation type uses anelectric power generation cell which is improved to work as a fuel celleven at low temperatures by lowering a resistance of an electrolyte, forexample, through making the electrolyte made of an yttria stabilizedzirconia (YSZ), be a thin film on the order of 10 μm in thickness.

A solid oxide fuel cell operable at high temperature is used for theseparator, for example, a ceramic having electronic conductivity such aslanthanum chromite (LaCrO₃), while a solid oxide fuel cell of a lowtemperature operation type can be used for the separator, i.e. ametallic material such as stainless steel.

Additionally, as structure of the solid oxide fuel cell, there have beenproposed three types, namely, a cylindrical type, a monolithic type anda flat plate type.

A stack of a solid oxide fuel cell has a structure in which electricpower generation cells, current collectors and separators arealternately laminated. A pair of separators sandwich an electric powergeneration cell from both sides of the cell in such a way that one ofthe separators is in contact with the air electrode through intermediaryof an air electrode current collector, while the other separator is incontact with the fuel electrode through intermediary of a fuel electrodecurrent collector. For the fuel electrode current collector, a spongyporous substance made of a Ni based alloy or the like can be used, whilealso for the air electrode current collector, a spongy porous substancemade of a Ag based alloy or the like can be used. A spongy poroussubstance simultaneously displays a current collection function, gaspermeation function, uniform gas diffusion function, cushion function,thermal expansion difference absorption function and the like, and isaccordingly suitable for a multifunction current collector.

The separators electrically connect between electric power generationcells, and also have a function to supply gas to the electric powergeneration cells. Therefore, each separator has a fuel path throughwhich fuel gas is introduced from a peripheral side of the separator andis discharged from a separator surface facing the fuel electrode layer,and an oxidant path through which oxidant gas is introduced from theperipheral side of the separator and is discharged from a separatorsurface facing the oxidant electrode layer.

<Problems to be Solved by the Invention>

<First Problem>

In a case of the solid oxide fuel cell of the low temperature operationtype, metal (stainless steel or the like) plates on the order of 5 to 10mm in thickness are used for the separators, and there has hitherto beenknown a separator having a structure such that gas discharge openings,to discharge fuel gas and oxidant gas introduced from the peripheralside of the separator into the current collector, are provided in acentral part of the separator.

FIG. 8 is a sectional view of a relevant portion of a fuel cell stackillustrating an example of the above described separator. In FIG. 8,reference numeral 3 denotes a fuel electrode layer, reference numeral 6denotes a fuel electrode current collector, reference numeral 8 denotesa separator, reference numeral 11 denotes a fuel path, reference numeral25 denotes a gas discharge opening, and arrows indicate a gas permeationcondition.

Here, it should be noted that such a conventional separator structure asdescribed above is associated with the following problems.

More specifically, the structure is such that fuel gas discharged fromthe central part of the separator 8 is supplied to an entire area of thefuel electrode layer 3 through the fuel electrode current collector 6made of a porous cushioning material; however, in practice, there is aproblem in that the fuel gas is consumed to a large extent by anelectrode reaction in a neighborhood of the gas discharge opening 25,and hence a gas concentration is decreased with increasing distance awayfrom the gas discharge opening 25. Consequently, the electrode reactionis not uniformly conducted over the entire area of the electrode, atemperature gradient is thereby generated in the electric powergeneration cell, the electric power generation cell is sometimes brokendown by thermal stress thus generated, and a resulting inefficientelectric power generation leads to degradation of electric powergeneration properties (electricity production comes to be large in acentral part of the electric power generation cell and small in aperipheral part of the same cell) . This problem has been particularlyconspicuous in the fuel electrode section.

Additionally, use of thick metallic plates of 5 to 10 mm in thicknessmakes a weight of a single cell itself great, and accordingly, in a caseof a solid oxide fuel cell constructed by longitudinally arranging cellstacks, there is a problem such that the electric power generation cellsin the cell stacks located in a bottom portion tend to be broken by aweight of the fuel cell. Consequently, as affairs stand, there remains aproblem in that a cell configuration is inevitably constrained in such away that a number of laminations is consistent with a tolerable weightof the fuel cell. Incidentally, in a case of a conventional structure, aweight of a cell stack is about 1 kg, and a total weight of a cellmodule made by laminating a large number of these cell stacks comes tobe about 25 kg. Consequently, a structure supporting such a module isnaturally complex.

<Second Problem>

As described above, in a conventional solid oxide fuel cell, each of thecurrent collectors made of a porous cushioning material is arrangedbetween an electrode layer and a separator, and gas is distributed to besupplied to each of the electrode layers through the current collectors;however, there has been a problem in that in the conventional structure,a retaining time of the gas in a current collector is short, andconsequently fuel gas not engaging with the electrode reaction isdischarged outside the electric power generation cell, so that electricpower generation efficiency is thereby degraded.

Additionally, in the conventional structure, a linear velocity of gas inthe peripheral part of the electric power generation cell comes to below. Consequently, there has also been a problem in that from theperipheral part of the electric power generation cell, air as oxidant istaken into an interior of the electric power generation cell, where acombustion reaction tends to take place, the combustion reactioncompletely consumes the fuel gas to be usable for the electrodereaction, and consequently electric power generation efficiency isdegraded.

Such an adverse phenomenon has remarkably taken place particularly in afuel cell stack provided with separators having a structure in whichfuel gas or oxidant gas is supplied to the fuel cell electrode currentcollector or the oxidant electrode current collector from a central partof each separator.

SUMMARY OF THE INVENTION

In view of the above described problems, a first object of the presentinvention is provision of a planar solid oxide fuel cell in whichelectric power generation efficiency is improved by uniformizing anelectrode reaction in current collectors, and adverse effects such asbreakdown accidents are prevented by making separators light in weight,and provision of a separator for use in the solid oxide fuel cell.

More specifically, the present invention according to a first aspect isa planar solid oxide fuel cell in which a fuel electrode layer and anoxidant electrode layer are arranged on both surfaces of a solidelectrolyte layer, respectively; a fuel electrode current collector andan oxidant electrode current collector are arranged outside the fuelelectrode layer and the oxidant electrode layer, respectively;respective separators are arranged outside the fuel electrode currentcollector and the oxidant electrode current collector; and a fuel gasand an oxidant gas are supplied from the respective separators to thefuel electrode layer and the oxidant electrode layer respectively,through the fuel electrode current collector and the oxidant electrodecurrent collector, respectively, with the fuel cell being characterizedin that each of the separators includes a first gas discharge openingfor discharging introduced gas from a central part of the separator anda plurality of second gas discharge openings for discharging theintroduced gas along a peripheral part of the separator in a circularmanner.

In the configuration described above, the gas is discharged from thecentral part of each separator and is discharged in a circular mannerfrom the peripheral part of each separator. Accordingly, the gas can besufficiently supplied to and distributed over entire areas of thecurrent collectors. Consequently, electrode reactions are caused to beperformed uniformly all over entire areas of the electrodes; thusefficient electric power generation can be performed in which adifference in electricity production between central parts andperipheral parts is eliminated.

Additionally, the present invention according to a second aspect ischaracterized in that in the planar solid oxide fuel cell according tothe first aspect, each separator is made by laminating a plurality ofthin metal plates at least including a thin metal plate provided withthe first gas discharge opening and the second gas discharge openings,and a thin metal plate with a worked indented surface.

According to the above described configuration, the separatorsthemselves can be made light in weight, concavities and convexities ofthe thin metal plates form gas flow paths, and hence introduced gas isdiffused uniformly over entire areas of the separators, so that ensuredis gas supply to the first gas discharge opening as a matter of courseand also to the second gas discharge openings formed in peripheral partsin a circular manner.

Additionally, the present invention according to a third aspect is aplanar solid oxide fuel cell according to the second aspect,characterized in that the thin metal plate provided with the first gasdischarge opening and the second gas discharge openings is arranged atleast on a side of each of the fuel electrode current collectors.

Nonuniformity of an electrode reaction in the current collectors isconspicuous around portions where supplied gas enters. This isascribable to the fact that in contrast to air (an oxidant gas), fuelgas cannot be supplied in a large amount, so that a supply amount isrestricted. Accordingly, in the present configuration, such gasdischarge structure as described above is applied at least to separatorportions in contact with the fuel electrode current collectors, so thatnonuniformity of an electrode reaction in the fuel electrode layers isreduced.

Additionally, the present invention according to a fourth aspect is aseparator for use in a solid oxide fuel cell which is contacted witheach current collector arranged outside each electrode to form a gaspassage for supplying a gas to the electrode, characterized in that theseparator includes a first gas discharge opening for discharging anintroduced gas from a central part thereof and a plurality of second gasdischarge openings for discharging the gas along a peripheral partthereof in a circular manner.

Additionally, the present invention according to a fifth aspect is theseparator for use in a solid oxide fuel cell according to the fourthaspect, characterized in that the separator is made by laminating aplurality of thin metal plates including at least the thin metal plateprovided with the first gas discharge opening and the second gasdischarge openings, and a thin metal plate having a worked indentedsurface.

Additionally, the present invention according to a sixth aspect is theseparator for use in a solid oxide fuel cell according to the fifthaspect, characterized in that the thin metal plate provided with thefirst gas discharge opening and the second gas discharge openings isarranged at least on a side of the fuel electrode current collector.

Furthermore, in view of the above described problems involved inconventional techniques, another object of the present invention isprovision of a solid oxide fuel cell in which electric power generationefficiency is improved by increasing utilization ratios of fuel gas andoxidant gas in current collectors, and provision of a separator for usein the solid oxide fuel cell.

More specifically, the invention according to a seventh aspect is asolid oxide fuel cell in which a fuel electrode layer and an oxidantelectrode layer are arranged on both surfaces of a solid electrolytelayer, respectively; a fuel electrode current collector and an oxidantelectrode current collector, with both collectors being formed of aporous substance, are arranged outside the fuel electrode layer and theoxidant electrode layer, respectively; respective separators arearranged outside the fuel electrode current collector and the oxidantelectrode current collector; and a fuel gas and an oxidant gas aresupplied from the respective separators to the fuel electrode layer andthe oxidant electrode layer, respectively, through the fuel electrodecurrent collector and the oxidant electrode current collector,respectively; with the fuel cell being characterized in that indents areformed on a surface of each of the separators, which surface is incontact with each of the current collectors, to increase a dwell volumeof gas in the current collectors.

In the above described configuration, the current collectors made of aspongy porous substance each are expanded in conformity with adepression of an associated separator, and hence volumes of theseparators are increased, so that a retaining time of the gas iselongated (a gas permeation rate is made lower) if a supplied amount ofthe gas is constant. In this way, a reaction between gases and theelectrode layers comes to be conducted satisfactorily, and electricpower generation efficiency is thereby improved.

Additionally, the invention according to an eighth aspect is a solidoxide fuel cell in which a fuel electrode layer and an oxidant electrodelayer are arranged on both surfaces of a solid electrolyte layer,respectively; a fuel electrode current collector and an oxidantelectrode current collector, with both collectors being formed of aporous substance, are arranged outside the fuel electrode layer and theoxidant electrode layer, respectively; respective separators arearranged outside the fuel electrode current collector and the oxidantelectrode current collector; and a fuel gas and an oxidant gas aresupplied from the respective separators to the fuel electrode layer andthe oxidant electrode layer, respectively through the fuel electrodecurrent collector and the oxidant electrode current collector,respectively; with the fuel cell being characterized in that aperipheral part of a surface of each of the separators, which surface isin contact with each of the current collectors, is protruded expandablyto increase linear velocities of gases in peripheral parts of thecurrent collectors.

An increase of the linear velocity of the gas being discharged in theperipheral parts prevents air entrained from the peripheral parts, andin particular, in peripheral parts of the fuel electrode layers, canmaintain a fuel gas concentration in an elevated concentrationcondition, and electric power generation performance is therebyimproved.

Additionally, the invention according to a ninth aspect is a solid oxidefuel cell in which a fuel electrode layer and an oxidant electrode layerare arranged on both surfaces of a solid electrolyte layer,respectively; a fuel electrode current collector and an oxidantelectrode current collector, with both collectors being formed of aporous substance, are arranged outside the fuel electrode layer and theoxidant electrode layer, respectively; respective separators arearranged outside the fuel electrode current collector and the oxidantelectrode current collector; and a fuel gas and an oxidant gas aresupplied from the respective separators to the fuel electrode layer andthe oxidant electrode layer, respectively, through the fuel electrodecurrent collector and the oxidant electrode current collector,respectively; with the fuel cell being characterized in that indents areprovided on a surface of each of the separators, which surface is incontact with each of the current collectors, and a peripheral part ofthe separator is protruded expandably.

In the above described configuration, a gas permeation rate in theinterior of the current collectors is made low and electrode reactionsare made satisfactory, and a linear velocity of gas in peripheral partsis made large, and entrainment of air from peripheral parts can therebybe prevented. Consequently, electric power generation performance can beimproved.

Additionally, the present invention according to a tenth aspect is thesolid oxide fuel cell according to any one of the seventh to ninthaspects, characterized in that a surface shape of the separators isformed at least on surfaces in contact with the current collectors.

A phenomenon of an incomplete reaction of gas in an interior of thecurrent collectors takes place on portions where supplied fuel gasenters. This is ascribable to the fact that in contrast to air (anoxidant gas), the fuel gas cannot be supplied in large amount, so that asupply amount thereof is restricted. Accordingly, in the presentconfiguration, depressions and protruded portions are provided at leaston the surface, in contact with one of the fuel electrode currentcollectors, of each of the separators, and the phenomenon of theincomplete reaction of the gas and a phenomenon of entrainment of air inthe fuel electrode current collector are thereby remedied.

Additionally, the invention according to an eleventh aspect is the solidoxide fuel cell according to any one of the seventh to tenth aspects,characterized in that the fuel cell includes a structure in which thefuel gas and the oxidant gas are supplied from central parts of theseparators, respectively, to the fuel electrode layer and the oxidantelectrode layer, respectively, through the fuel electrode currentcollector and the oxidant electrode current collector, respectively.

Additionally, the invention according to a twelfth aspect is a separatorfor use in a solid oxide fuel cell which is in contact with one of thecurrent collectors arranged outside the respective electrodes to form agas passage for supplying a gas to one of the electrode sections,characterized in that indents are provided on a surface of thisseparator, which surface is in contact with one of the currentcollectors, to increase a dwell volume of gas in the current collectors.

Additionally, the invention according to a thirteenth aspect is aseparator for use in a solid oxide fuel cell which is contacted witheach current collector arranged outside each electrode to form a gaspassage for supplying a gas to each electrode section, characterized inthat a peripheral part of a surface of the separator, which surface isin contact with the current collector, is protruded expandably toincrease a linear velocity of gas in a peripheral part of the currentcollector.

Additionally, the invention according to a fourteenth aspect is aseparator for use in a solid oxide fuel cell which is contacted witheach current collector arranged outside each electrode to form a gaspassage for supplying a gas to each electrode section, characterized inthat indents are provided on a surface of the separator, which surfaceis in contact with the current collector, and a peripheral part of thesurface concerned is protruded expandably.

Additionally, the invention according to a fifteenth aspect is theseparator according to any one of the twelfth to fourteenth aspects,characterized in that a surface shape of the separator is formed atleast on a surface in contact with one of the fuel electrode currentcollectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded oblique perspective view illustrating aconfiguration of a relevant portion of a planar solid oxide fuel cellinvolved in the present invention;

FIG. 2 a and FIG. 2 b illustrate a structure of a separator on a side ofa fuel electrode involved in the present invention; with FIG. 2 a beinga related plan view and FIG. 2 b being a related sectional view;

FIG. 3 is a sectional view of a relevant portion of a fuel cell stackinvolved in the present invention;

FIG. 4 is a sectional view of a relevant portion of a fuel cell stackillustrating a shape of a separator according to a second embodiment ofthe present invention;

FIG. 5 a to FIG. 5 d are sectional views illustrating shapes ofseparators different from a shape shown in FIG. 1;

FIG. 6 is an exploded sectional view of a solid oxide fuel cell;

FIG. 7 is an exploded perspective view of a relevant portion of thesolid oxide fuel cell; and

FIG. 8 is a sectional view of a relevant portion of a conventional fuelcell stack.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description will be made below of embodiments of the present inventionwith reference to the accompanying drawings. Incidentally, in thefollowing description, for simplification of description, the samereference symbols are used for portions common to conventional portions.

First Embodiment

Description will be made below of a first embodiment of the presentinvention with reference to FIG. 1, FIG. 2 a to FIG. 2 b, and FIG. 3;initially, on the basis of FIG. 1, description will be made on aconfiguration of a solid oxide fuel cell involved in the presentembodiment.

In FIG. 1, reference numeral 1 denotes a fuel cell stack, which has astructure in which an electric power generation cell 5, in which a fuelelectrode layer 3 and an air electrode layer (oxidant electrode layer) 4are arranged respectively on both surfaces of a solid electrolyte layer2, a fuel electrode current collector 6 arranged outside the fuelelectrode layer 3, an air electrode current collector (oxidant electrodecurrent collector) 7 arranged outside the air electrode layer 4, andseparators 8 arranged respectively outside the current collectors 6 and7 are laminated in this order. The present embodiment is suitablyapplicable to a sealless structure in which no gas seal is present alonga rim of a fuel electrode current collector.

Here, the solid electrolyte layer 2 is formed of a stabilized zirconia(YSZ) that is added with yttria and the like, the fuel electrode layer 3is formed of a metal such as Ni or Co, or a cermet such as Ni—YSZ orCo—YSZ, the air electrode layer 4 is formed of LaMnO₃, LaCoO₃ or thelike, the fuel electrode current collector 6 is formed of a spongyporous sintered metal plate made of a Ni based alloy or the like, andthe air electrode current collector 7 is formed of a spongy poroussintered metal plate made of a Ag based alloy or the like.

The separators 8 have a function to connect electrically betweenelectric power generation cells 5 similarly to conventional separators,and also have a function to supply a gas to the electric powergeneration cells 5; however, a structure of the separators is differentfrom a structure of conventional separators shown in FIG. 8.

More specifically, a conventional separator is fabricated of a thick,single metal plate, whereas as shown in FIG. 2 a and FIG. 2 b, separator8 of the present embodiment has a three layer structure which is formedby successively laminating a metal upper plate 21 provided with aplurality of gas discharge openings, an intermediate plate 22 processedto have a surface with alternate convexities and concavities, and a flatlower plate 23. For all these plates, thin metal plates made ofstainless steel or the like are used.

In the upper plate 21, a first fuel gas discharge opening 25 is formedin a central part thereof, and a plurality of second fuel gas dischargeopenings 24 are formed in a circularly aligned manner. Fuel gasintroduced from a rim face of the separator 8 is discharged, through afuel gas passage 11, from these gas discharge openings 24 and 25, andsupplied to the fuel electrode current collector 6 facing the separator8.

For the intermediate plate 22, there is used a sheet metal materialprocessed so as to have a surface with alternate convexities andconcavities for a purpose of ensuring strength and a thickness as aseparator. This plate is combined with the upper plate 21 and the lowerplate 23 to form a hollow separator 8 as shown in FIG. 2 b. Hollowportions formed by these convexities and concavities function as a gasflow path allowing fuel gas to diffuse easily, and simultaneously aweight savings of the separator 8 can be actualized.

Incidentally, a worked indented surface can be formed by performingplastic working on this sheet metal. In contrast to a rectangular shapeshown in FIG. 2 b, a corrugated shape (corrugated plate) may also beused. Additionally, a plate material provided with worked indentedsurface patterns by performing embossing processing may also be used.

The lower plate 23 forms a partition wall between a fuel electrodesection and an air electrode section. The above described combination ofthe upper plate 21 and the intermediate plate 22 constitutes a separatorstructure on a fuel electrode side. In practice, a separator portion onan air electrode side is formed with an intervening lower plate 23, butin the figure concerned, a relevant portion is omitted.

Incidentally, the separators 8 (8A, 8B) at both ends of the fuel cellstack 1 shown in FIG. 1 have respectively either one of the abovedescribed separator structures on the fuel electrode side and the airelectrode side.

In the above described configuration of the planar solid oxide fuelcell, fuel gas discharged from a central part and a peripheral part ofthe separator 8 can be spread over an entire area of the fuel electrodelayer 3 with a satisfactory distribution through the fuel electrodecurrent collector 6. Accordingly, gas reaction can be performedefficiently over the entire area of the electrode layer.

More specifically, a conventional type separator, provided with gasdischarge opening 25 merely in a central part of the separator 8 shownin FIG. 8, has a structure such that gas can be hardly spread to aperipheral part, and accordingly, an electrode reaction is not spatiallyuniform, so that there have been caused problems including breakdown ofan electric power generation cell and degradation of electric powergeneration efficiency due to thermal stress; however, according to theseparator structure of the present embodiment, as shown in FIG. 3, fuelgas introduced from a peripheral face of the separator through the fuelpath 11 is made to diffuse over an entire area of the separator bytaking advantage of the hollow portions (convexities and concavities) ofthe separator 8 as the gas passage. The fuel gas is discharged from thefirst fuel gas discharge opening 25 in the central part and the aplurality of second fuel gas discharge openings 24 in the peripheralpart, and the fuel gas can be spread over the entire area of the fuelelectrode layer 3 with a satisfactory distribution through the fuelelectrode current collector 6 facing the separator. Consequently, anelectrode reaction comes to be performed uniformly over entire electrodeareas, and hence electric power generation can be performed efficientlywith a vanishing difference in electricity production between thecentral part and the peripheral part.

Moreover, the separator 8 of the present embodiment is made to have alaminate structure with a hollow interior, and hence a weight of theseparator itself can be drastically reduced as compared to theconventional type separator. Such a structure is extremely effective ina fuel cell module having a structure in which a large number of cellstacks are longitudinally laminated, in view of the fact that a burdenloaded on electric power generation cells located in lower positions isreduced. Consequently, a supporting frame for the fuel cell module canbe simplified, and a constraint imposed on a number of laminations in acell stack can be alleviated. Thus, an electric power generation of highelectromotive force can be actualized.

As described above, as for the present embodiment, description has beenmade on the structure of the separator part in contact with the fuelelectrode current collector 6, and a similar structure can be applied tothe separator part in contact with the air electrode current collector7. Additionally, some simple discharge structure other than thosedescribed above (for example, as shown in FIG. 7, a gas dischargestructure restricted to the central part) can be adopted. Nonuniformityof electrode reaction in an interior of the current collectors isconspicuous around portions where supplied fuel gas enters, andaccordingly, it is important to apply the structure of the presentembodiment at least to the separator part facing the fuel electrodecurrent collector 6.

Additionally, in the present embodiment, the separator 8 has a threelayer structure formed of three thin metal plates; however, theseparator structure is not restricted to this structure, and may take atwo layer structure in which the lower plate 23 is omitted. In this way,a further weight savings of the separator 8 can be expected.

Additionally, in the present embodiment, there is presented a solidoxide fuel cell in which a stabilized zirconia (YSZ) that is added withyttria is used for an electrolyte in the electric power generation cell;however, the present invention can be applied to other solid oxide fuelcells such as those solid oxide fuel cells in which a ceria basedelectrolyte and a gallate based electrolyte are used.

Second Embodiment

Now, description will be made below of a second embodiment of thepresent invention. FIG. 4 shows a sectional view of a relevant portionof a fuel cell stack illustrating a shape of a separator, FIG. 5 a toFIG. 5 d show sectional views of relevant portions illustrating otherexamples of separators, FIG. 6 shows an exploded sectional view of asolid oxide fuel cell, and FIG. 7 shows an exploded oblique perspectiveview of a relevant portion of the same solid oxide fuel cell in thepresent embodiment.

Initially, on the basis of FIG. 6 and FIG. 7, description will be madebelow of a configuration of the solid oxide fuel cell involved in thepresent embodiment.

In FIG. 6, reference numeral 1 denotes a fuel cell stack, which has astructure in which an electric power generation cell 5 in which a fuelelectrode layer 3 and an air electrode layer (oxidant electrode layer) 4are arranged respectively on both surfaces of a solid electrolyte layer2, a fuel electrode current collector 6 arranged outside the fuelelectrode layer 3, an air electrode current collector (oxidant electrodecurrent collector) 7 arranged outside the air electrode layer 4, andseparators 8 arranged respectively outside the current collectors 6 and7 are laminated in this order.

The solid electrolyte layer 2 is formed of a stabilized zirconia (YSZ)that is added with yttria and the like, the fuel electrode layer 3 isformed of a metal such as Ni or Co, or a cermet such as Ni—YSZ orCo—YSZ, the air electrode layer 4 is formed of LaMnO₃, LaCoO₃ or thelike, the fuel electrode current collector 6 is formed of a spongyporous sintered metal plate made of a Ni based alloy or the like, theair electrode current collector 7 is formed of a spongy porous sinteredmetal plate made of a Ag based alloy or the like, and the separators 8are formed of a stainless steel or the like.

Here, the porous metal plates forming the current collectors 6 and 7 areplates having been fabricated through performance of the followingsteps. The order of the steps is as follows: a step for preparing aslurry→a step for molding→a step for foaming→a step for drying→a stepfor degreasing→a step for sintering.

Initially, in the step for preparing a slurry, a metal powder, anorganic solvent (n-hexane or the like), a surfactant (sodiumdodecylbenzenesulfonate or the like), a water soluble resin binder(hydroxypropylmethyl cellulose or the like), a plasticizer (glycerin orthe like) and water are mixed together, and thus a foaming slurry isprepared. In the step for molding, by use of a doctor blade method, theslurry is molded in a thin plate shape on a carrier sheet, and thus agreen sheet is obtained. Then, in the step for foaming, this green sheetis foamed into a spongy condition in a high temperature and highhumidity environment with aid of vapor pressure of a volatile organicsolvent and a foaming property of the surfactant. Subsequently, a porousmetal plate is obtained through the step for drying, the step fordegreasing and the step for sintering.

In this case, in the step for foaming, bubbles generated in the greensheet grow with nearly spherical shapes as a result of receiving nearlyequivalent pressures along all directions. When a bubble diffuses toapproach an interface with an atmosphere, the bubble grows toward a thinpart of the slurry interposed between the bubble and the atmosphere, andeventually the bubble is broken and gas inside the bubble diffuses intothe atmosphere through formed small holes. Accordingly, there isobtained a porous metal plate provided with continuous pores havingopenings on its surface. The current collectors 6 and 7 each are formedby cutting a thus fabricated porous metal plate having a threedimensional skeleton structure, into a circular form.

On the other hand, as shown in FIG. 6 and FIG. 7, the separators 8electrically connect between electric power generation cells 5, and alsohave a function to supply gas to the electric power generation cells 5.Therefore, each separator has a fuel path 11 through which fuel gas isintroduced from a peripheral side of the separator 8 and is dischargedfrom an approximately central part of a surface of the separator 8facing the fuel electrode current collector 6, and an oxidant path 12through which oxidant gas is introduced from the peripheral side of theseparator 8 and is discharged from a separator surface facing the airelectrode current collector 7. Here, it should be noted that theseparators 8 (8A, 8B) at both ends of the stack have respectively eitherone of the paths 11 and 12.

Additionally, the separator 8 of the present embodiment is differentfrom a flat shaped conventional type shown in FIG. 8 in that a surfaceof the separator 8 in contact with the fuel electrode current collector6 is made to be bowl shaped, as shown in FIG. 4, by providing adepression 8 a with a deepened central part, and consequently, asituation is such that peripheral part 8 b is raised. As has alreadybeen described, material for the fuel electrode current collector 6itself is formed of a spongy foam, and hence, at a time of lamination,the foam is arranged in a condition such that the foam is in closecontact with a depression shape of the separator 8. Therefore, as far asthe separator 8 shown in FIG. 4 is used, the fuel electrode currentcollector 6 is made to have a shape in which a central part of thecollector is swollen as compared to a conventional collector (forexample, if a thickness of a conventional fuel electrode currentcollector 6 is about 0.75 mm, a maximum thickness of the central part ismade to increase on the order of about 1.5 mm the case of the presentembodiment), and moreover, the peripheral part is made to be thinner ascompared to a conventional type (for example, made to be on the order of0.2 mm in relation to a thickness of 0.75 mm of the conventional type).

Additionally, as shown in FIG. 6, respectively on both sides of the fuelcell stack 1, a manifold 15 for fuel for supplying fuel gas throughconnecting pipes 13 to fuel paths 11 in respective separators 8, and amanifold 16 for oxidant for supplying oxidant gas through connectingpipes 14 to oxidant paths 12 in the respective separators 8, arearranged along a direction of lamination of the electric powergeneration cells 5 in an extended manner.

According to the above described configuration of the fuel cell, fuelgas discharged from the central part of the separators 8 is spread overan entire area of the fuel electrode layer 3 through the fuel electrodecurrent collector 6 with a satisfactory distribution, and thus asatisfactory gas reaction can be performed over the entire area of theelectrode layer.

More specifically, as shown in FIG. 8, in a conventional type havingflat separators 8, fuel electrode current collectors 6 are also flatshaped, and in particular, a permeation rate of fuel gas (arrows in thisfigure) is high in a neighborhood of a central part of each of the fuelelectrode current collectors 6 (in other words, a retaining time of gasin the current collector is short) . Thus, an electrode reaction in theneighborhood of the central part of the electrode layer is notcompletely performed, and moreover, a situation is such that the gas isnot sufficiently spread to a peripheral part, so that nonuniformity ofthe electrode reaction is caused, and there is a possibility such thatmost of the fuel gas not engaged in the reaction is vainly dischargedoutside the electric power generation cell. On the contrary, use of theseparators 8 shown in FIG. 4 increases a volume of the fuel electrodecurrent collectors 6 themselves, so that if a supplied amount of gasfrom the separators 8 is constant, a permeation rate of the gas is thereby made lower and a retaining time of the gas in the current collectorscan be made longer. Consequently, gas discharged from the central partof each of the separators 8 can be made to permeate a wide area from thecentral part to the peripheral part of the fuel electrode currentcollector 6, and the fuel gas can thereby be supplied to the fuelelectrode layer 3 in a uniformly distributed manner, so that asatisfactory gas reaction can be performed over the entire area of theelectrode layer.

Additionally, in each of the separators 8 of the present embodiment, theperipheral part is protruded expandably, and a thickness of theperipheral part of the fuel electrode current collector 6 thereby comesto be thinner than a corresponding thickness of the conventional type.Therefore, particularly in a case of a sealless structure (a type inwhich a rim of the fuel electrode current collector has no gas seal), alinear velocity of gas being discharged is increased in the peripheralpart of the fuel electrode current collector, and entrainment of airfrom the peripheral part is thereby prevented and a combustion reactionin an interior of the electric power generation cell can be inhibited,so that also in the peripheral part of the fuel electrode layer 3, therecan be maintained a condition in which a fuel gas concentration israised, and an improvement of electric power generation performance canthereby be expected.

As described above, as for the present embodiment, description has beenmade of a shape of the surface, in contact with fuel electrode currentcollector 6, of the separator 8. A shape of the surface, in contact withthe air electrode current collector 7, of the separator 8 can be made tohave a similar shape. Additionally, the shape of the surface of theseparator 8 is not limited to the shape shown in FIG. 4, and variousshapes as shown in FIG. 5 a to FIG. 5 d are conceivable. In thesefigures, reference numeral 8 a denotes a depression located in a centralpart or in a neighborhood thereof similarly to the case described above,reference numeral 8 b denotes a peripheral part raised along a peripheryof the depression 8 a. To sum up, acceptable is a shape in which avolume of the current collector can be made larger, and thickness of theperipheral part can be made small.

Additionally, as the porous structure of the current collectors 6 and 7,mesh, felt and the like can be used in addition to foam.

Additionally, in the present embodiment, there is presented a solidoxide fuel cell in which a stabilized zirconia (YSZ) that is added withyttria is used for the electrolyte in the electric power generationcell; however, the present invention can be applied to other solid oxidefuel cells such as those solid oxide fuel cells in which a ceria basedelectrolyte and a gallate based electrolyte are used.

INDUSTRIAL APPLICABILITY

<Effect of the First Embodiment>

As described above, according to the present invention set forth in thefirst and fourth aspects, gas discharge openings are provided in thecentral part and the peripheral part of a separator, so that gas can besufficiently spread over an entire area of a current collector.Consequently, an electrode reaction can be performed uniformly over anentire area of the electrode, and thus an efficient electric powergeneration can be performed in which a difference in electricityproduction between the central part and the peripheral part of theelectric power generation cell is eliminated.

Additionally, according to the present invention set forth in the secondand the fifth aspects, the separators are made by laminating a pluralityof thin metal plates including at least the thin metal plates eachprovided with a first gas discharge opening and second gas dischargeopenings, and thin metal plates having a worked indented surface.Consequently, the separators themselves are made light in weight, and anumber of laminations of a cell stack in a longitudinal type fuel cellmodule can thereby be increased, so that an electric power generation ofhigh electromotive force can be actualized. Additionally, convexitiesand concavities form the gas flow path, and hence introduced gas comesto be easily supplied to the entire area of the current collector, sothat an efficient electric power generation can be actualized in whichnonuniformity of an electrode reaction in the interior of the currentcollector is reduced.

Additionally, according to the present invention set forth in the thirdand sixth aspects, the above described separator structure according tothe first and second aspects is applied at least to a separator part onthe side of the fuel electrode current collector, so that anonuniformity phenomenon of an electrode reaction in the interior of thefuel electrode current collector, which is conspicuous around portionswhere the supplied gas enters, can be effectively improved, andconsequently an efficient electric power generation can be actualized inwhich a fuel utilization ratio is high.

<Effect of the Second Embodiment>

Additionally, according to the invention set forth in the seventh andtwelfth aspects, indents are provided on the surface, in contact withone of the current collectors, of each of the separators, andaccordingly, a dwell volume of gas in the interior of the currentcollectors is increased, and hence a retaining time of the gas isthereby made longer (a gas permeation rate is made lower). Consequently,the gas is slowly spread over a wide area through the current collector,a satisfactory gas reaction comes to be performed over the entire areaof the electrode layer. Accordingly, a fuel utilization ratio and an airutilization ratio are increased, and electricity generation performanceis improved.

Additionally, according to the invention set forth in the eighth andthirteenth aspects, the peripheral part of the surface, in contact withthe current collector, of the separator is protruded expandably, andaccordingly, a linear velocity of gas being discharged is raised in aperipheral part, entrainment of air from the peripheral part isprevented, and a combustion reaction in the interior of the electricpower generation cell can be inhibited. Consequently, in the peripheralpart of the fuel electrode layer, there can be formed a condition inwhich a fuel gas concentration is raised, and electric power generationperformance is thereby improved.

Additionally, according to the invention set forth in the ninth andfourteenth aspects, indents are provided on the surface, in contact withthe current collector, of the separator, and the peripheral part of theseparator is protruded in an expanded manner. Therefore, effects setforth in the first and second aspects are obtained in which a permeationrate of gas in the interior of the current collector is made lower andan electrode reaction is made satisfactory; moreover, a linear velocityof gas being discharged in the peripheral part is made great, andentrainment of air from the peripheral part can be prevented.

Additionally, according to the invention set forth in the tenth andfifteenth aspects, the above described surface shape of the separator ismade to be formed at least on the surface thereof in contact with thefuel electrode current collector, so that the phenomena of an incompletereaction of the gas and the entrainment of air in the fuel electrodecurrent collector are improved without failure, and hence electric powergeneration performance is improved.

Additionally, according to the invention set forth in the eleventhaspect, the structure is such that gases are supplied respectively fromcentral parts of the separators to the fuel electrode layer and theoxidant electrode layer, respectively, through the fuel electrodecurrent collector and the oxidant electrode current collector.Therefore, gases slowly permeate over wide areas from the central partsof the current collectors to the peripheral parts, and supplied to theelectrode layers in a uniformly distributed manner, and satisfactoryelectrode reactions come to be performed over the entire areas of theelectrode layers.

1. An oxide fuel cell comprising: a fuel electrode layer and an oxidantelectrode layer on opposite surfaces of a solid electrolyte layer,respectively; a porous fuel electrode current collector and a porousoxidant electrode current collector positioned outside said fuelelectrode layer and said oxidant electrode layer, respectively; a firstseparator, positioned outside said porous fuel electrode currentcollector, for allowing a fuel gas to be supplied from said firstseparator to said fuel electrode layer through said porous fuelelectrode current collector, said first separator having a surface incontact with said porous fuel electrode current collector; and a secondseparator, positioned outside said porous oxidant electrode currentcollector, for allowing an oxidant gas to be supplied from said secondseparator to said oxidant electrode layer through said porous oxidantelectrode current collector, said second separator having a surface incontact with said porous oxidant electrode current collector, wherein aperipheral part of said surface of said first separator protrudestowards a peripheral part of said porous fuel electrode currentcollector so that a thickness of said peripheral part of said porousfuel electrode current collector is smaller than a thickness of anyother portion of said porous fuel electrode current collector to therebyincrease a linear velocity of the fuel gas in said peripheral part ofsaid porous fuel electrode current collector, and a peripheral part ofsaid surface of said second separator protrudes towards a peripheralpart of said porous oxidant electrode current collector so that athickness of said peripheral part of said porous oxidant electrodecurrent collector is smaller than a thickness of any other portion ofsaid porous oxidant electrode current collector to thereby increase alinear velocity of the oxidant gas in a peripheral part of said porousoxidant electrode current collector.
 2. The oxide fuel cell according toclaim 1, wherein said first separator is for allowing a fuel gas to besupplied from said first separator to said fuel electrode layer throughsaid porous fuel electrode current collector by allowing the fuel gas tobe supplied from a central part of said first separator, through saidporous fuel electrode current collector and to said fuel electrodelayer, and said second separator is for allowing an oxidant gas to besupplied from said second separator to said oxidant electrode layerthrough said porous oxidant electrode current collector by allowing theoxidant gas to be supplied from a central part of said second separator,through said porous oxidant electrode current collector and to saidoxidant electrode layer.
 3. The oxide fuel cell according to claim 1,wherein each of said porous fuel electrode current collector and saidporous oxidant electrode current collector is made from a spongy porousmetal material.